1. Field of the Invention
The present invention relates to signal generators in general and in particular to a signal generator for providing symmetrical complex waveforms having a predetermined harmonic content and closely matched rising and falling edges.
2. Description of the Prior Art
Various techniques have been used in the past for generating complex waveforms having a selected harmonic content.
Among the various techniques which have been used, one well known technique, called subtractive synthesis, comprises a plurality of analog filters. Beginning with a harmonically rich waveform, such as a squarewave or sawtooth, the filters, comprising RC networks, filter out selected frequencies and/or bands of frequencies from the output of an oscillator to produce a desired waveform. While widely used, it is some times difficult to achieve desired waveforms using analog filters because such filters are typically sensitive to manufacturing and/or process variations.
Another well known technique comprises a plurality of individual ramp generators. A ramp generator provides an output, either current or voltage, which ramps either up or down from a predetermined initial magnitude to a predetermined final magnitude in a linear fashion.
In the past, it has been the practice to operate the ramp generators in sequence such that when one of the ramp generators has reached its predetermined final magnitude, a second ramp generator is triggered to provide its output. When the second ramp generator has reached its final magnitude, a third ramp generator is triggered to provide its output, and so on, until the desired waveform is provided. The number of ramp generators required and the slope and duration of each of their outputs depends on the complexity, i.e. harmonic content, of the waveform desired.
The above-described prior known use of ramp generators requires that the final and initial outputs of each of the ramp generators in the sequence be exactly matched and that the trigger signals be timed precisely so as not to produce undesired inflections and discontinuities in the complex waveform being generated. Such discontinuity-free operation is difficult to achieve especially when a large number of ramp generators is required.